In the integrated circuit industry today, hundreds of thousands of semiconductor devices are built on a single chip. Every device on the chip must be electrically isolated to ensure that it operates independently without interfering with another. With the high integration of the semiconductor devices, the accuracy of formation of feature patterns overlying a previously defined semiconductor device level is increasingly difficult as critical dimensions shrink. Overlay accuracy, also referred to as registration is critical to proper functioning of a semiconductor device. To successfully pattern an overlying feature level on the wafer, the wafer feature pattern must be accurately aligned with a newly applied pattern image included in a reticle for proper transfer of the image to the photoresist layer on the wafer. A reticle is used in step-and-repeat and step-and-scan optical exposure systems common in the art of semiconductor device fabrication to transfer feature patterns to individual die on the wafer surface in a multi-level semiconductor device.
In forming the various levels of a multi-level semiconductor device including shallow trench isolation features, semiconductor wafer alignment for positioning the semiconductor wafer, for subsequent device feature patterning is critical. In a typical photolithographic patterning procedure, an automated stepper, for example, an ASM Lithography photo system sequentially positions the wafer beneath a photoimaging system for transferring a patterned photoimage of device features formed on a reticle to expose a photoresist material overlying the semiconductor wafer surface. As positioning of the process wafer is critical for forming semiconductor features, methods for forming alignment marks in the semiconductor wafer surface have evolved to allow the automated stepper to optically sense the alignment marks for proper process wafer positioning.
Several wafer alignment strategies exist for using different patterns and locations to achieve the alignment of a semiconductor wafer to a reticle containing an image to be transferred to the wafer. These strategies vary from alignment marks located between shot sites (also known as chip sites) to global alignment marks located in two shot sites at the periphery of the wafer. There are also global strategies in which the alignment marks are located between shot sites in the more peripheral regions of the wafer. The overlay accuracy required for proper alignment, frequently referred to as an overlay budget is about one third of the critical dimension. As device technologies scale to about 0.10 microns and below, conventional methods for forming and replicating alignment marks are no longer sufficiently accurate.
In one approach for global alignment, at least two areas at the wafer periphery are selected, typically located on opposite sides of the wafer diameter and include a series of parallel scribe marks covering a rectangular or square area of about 50 square microns to about 400 square microns referred to as zero-level alignment marks that are etched into the silicon before other processing steps. The global alignment marks are subsequently replicated in each subsequent level of manufacturing the multi-level semiconductor device.
Shallow trench isolation (STI), is a preferred electrical isolation technique especially for a semiconductor chip with high integration. STI features can be made using a variety of methods including, for example, the Buried Oxide (BOX) isolation method for shallow trenches. The BOX method involves filling the trenches with a chemical vapor deposition (CVD) silicon oxide (SiO2), also referred to as an STI oxide which is then chemically mechanically polished (CMP) to remove the overlying layer of STI oxide to yield a planar surface. The shallow trenches etched for the BOX process are anisotropically plasma etched into the substrate, for example, silicon, and are typically between 0.3 and 1.0 microns deep.
Shallow trench isolation features with trenches having submicrometer dimensions are effective in preventing latch-up and punch-through phenomena. Broadly speaking, conventional methods of producing a shallow trench isolation feature include: forming a hard mask, for example silicon nitride, over the targeted trench layer, for example including a pad oxide layer, patterning a photoresist over the hard mask to define a trench feature, anisotropically etching the hard mask to form a patterned hard mask, and thereafter anisotropically etching the trench feature to form the shallow trench isolation feature. Subsequently, the photoresist is removed (e.g., stripped) and the shallow trench isolation feature is back-filled, with a dielectric material, for example a CVD silicon oxide, also referred to as an STI oxide.
In processes according to the prior art, for example, associated with trench isolation manufacture, a first layer of insulating dielectric, often referred to as an STI oxide, for example, silicon oxide, is formed to backfill trenched areas formed in a silicon substrate to form shallow trench isolation features around active areas of the device, During the various material depositions and planarization steps the wafer peripheral areas including areas containing alignment marks (alignment mark areas) reflect various global material deposition and planarization steps that are carried out to form semiconductor devices in the active areas of the process wafer. As a result of active area processing steps the alignment mark areas include a layer of the STI oxide.
To preserve replication of the alignment marks for use in subsequent photolithographic patterning steps, in one approach in the prior art, an etching process is carried out on the STI oxide to form a window area to surround the alignment mark areas and to expose the underlying alignment marks with the window area having a depth greater than a subsequent STI oxide CMP polishing depth. In a blind photo imaging process to replicate the alignment marks a manually assisted alignment process is used to position the wafer for photolithographic patterning of the alignment mark areas (fields) for a subsequent etching process to replicate the alignment marks. A wet or dry etching process is then used to etch away the STI oxide overlying the alignment mark areas. A CMP step is then carried out to planarize the wafer including removing excess STI oxide over the active areas of the device. Following the STI oxide CMP step, a layer of polysilicon is typically conformally deposited over the alignment mark areas in parallel with active area processing steps, thereby replicating the alignment marks.
One problem according to prior art methods of replicating alignment marks, is that the manually assisted method for blindly positioning the process wafer for alignment mark etching is not sufficiently accurate, frequently requiring the formation of a relatively wide clear out window surrounding the alignment mark areas to ensure exposure of the alignment marks during etching. In addition, the window areas formed to surround the alignment marks in the STI oxide etching step form a recessed area present during the subsequent STI oxide CMP step. As a result, in carrying out the STI oxide CMP step residual particles from the slurry and polishing surface tend to become entrained in the recessed area requiring extensive cleaning processes frequently including another photolithographic patterning and etching step to replicate or restore the optical contrast of the alignment marks. Further, the need for an additional photolithographic patterning and etching process to clear out the alignment mark areas significantly increases the processing time. Other shortcomings in the prior art methods for replicating alignment mark areas include the possibility of overpolishing the alignment mark area during the STI oxide CMP process, thereby reducing a thickness window of the underlying silicon nitride layer and leading to the potentiality of damage to the silicon substrate.
Another problem in prior art methods for replicating alignment marks is the relative contrast of the alignment marks which are detected by an auto-imaging system. For example, a Helium-Neon laser having a wavelength between about 500 nm to about 630 run and which neutrally affects photoresists, is typically used as the light source in an alignment process. Many dielectric films are transparent in this wavelength range thereby presenting little interference with reflections from an underlying alignment mark area, typically having a higher extinction coefficient material to provide contrast producing reflections. During the processing of several levels in a multi-level semiconductor device, the alignment mark areas frequently are covered with high extinction coefficient materials such as SiGe, silicon carbide (e.g., SIC), silicon oxynitride (e.g. SiON), metal salicides, polysilicon, and metallic layers. While the alignment mark trenches are frequently not completely covered thereby losing their definition, the sharpness of the definition is decreased. As overlay budgets approach 20 to 30 nanometers for 0.10 micron critical dimensions and lower, a small decrease in the definition of the alignment marks by overlayers of high extinction coefficient materials is increasingly detrimental to overlay accuracy. In many cases an additional step to photolithographically pattern and etch the alignment mark area is economically prohibitive in terms of process cycle time and material cost.
Therefore, there is a need in the semiconductor processing art to develop an improved method for selectively etching alignment mark areas to replicate alignment marks such that costly process wafer photolithographic and etching process cycles may be avoided while improving high definition alignment mark optical contrast for improved overlay accuracy.
It is therefore an object of the invention to provide an improved method for selectively etching alignment mark areas to replicate alignment marks such that costly process wafer photolithographic and etching process cycles may be avoided while improving high definition alignment mark optical contrast for improved overlay accuracy including overcoming other shortcomings of the prior art.